Combined X-ray imaging apparatuses, capable of performing according to multiple imaging modes, are well known in the state of the art.
A dual purpose X-ray imaging apparatus equipped with a single sensor and capable of multiple imaging modes, namely partial CT (computerized tomography) imaging additional to panoramic imaging, for use in dental and medical diagnosis, is known from U.S. Pat. No. 6,118,842.
Another apparatus providing at least one x-ray detector which can be positioned in multiple imaging positions, and accordingly exposed by a collimated x-ray beam, depending on a selected imaging mode, is known from U.S. Pat. No. 6,055,292. The embodiment of a single movable case containing two x-ray detectors which can be positioned depending on imaging mode is further known from U.S. Pat. No. 7,559,692 and U.S. Pat. No. 7,798,708.
EP 1752099 teaches about a combined panoramic and computed tomographic apparatus characterized by a X-ray sensor part equipped with dual X-ray detectors and rotatable around an eccentric axis, so that, depending on the selected imaging mode, a different X-ray detector is placed in the X-ray beam at a preferred distance from the source and is exposed to radiation.
EP 1752100 teaches about a combined panoramic and computed tomographic apparatus characterized by a X-ray sensor part equipped with dual X-ray detector mounts on the rotating arm, displaced at different distance from the source. The X-ray detector mount for panoramic imaging is located closer to the source and can slide out of the beam when CT imaging mode is selected.
EP 1752099 teaches about a combined panoramic, computed tomography and cephalometric apparatus characterized by a X-ray sensor part equipped with dual X-ray detectors either rotatable around an axis, or displaceable by sliding, so that, depending on the selected imaging mode, a different X-ray detector is placed in the X-ray beam at a preferred distance from the source and is exposed to radiation.
U.S. Pat. No. 7,783,002 teaches about a combined panoramic and computed tomographic apparatus characterized by a fixed CT X-ray detector and a movable panoramic X-ray detector having a pivoting movement to turn it out of the beam when CT imaging mode is selected.
WO 2010/128404 teaches about a combined panoramic, computed tomography and cephalometric apparatus having an extended rotary arm and providing various arrangements for detector positioning in and out of the x-ray beam at predefined distances from the source, either by translation of a movable platen, or by rotation around a pivot axis, including a third position where both a panoramic and a CT detector are out of beam and a third detector for cephalography is exposed.
Concerning the cephalometric imaging mode U.S. Pat. No. 5,511,106 teaches a method of scanning the patient skull by a secondary collimator located in proximity of the patient and translating with a linear movement synchronized with the movement of the detector.
U.S. Pat. No. 7,103,141 teaches a method of modulating the radiation intensity during the cephalometric scanning process, either by adjustment of the tube voltage, or the tube current, or the scanning speed.
US 2009/168966 teaches about a combined apparatus having multiple imaging modes, to among which computed tomography, where the irradiation field is changed according to the selected imaging mode by a primary and secondary collimation mechanism; in particular, various collimator arrangements are described where the secondary collimator is essentially mentioned for proximal limitation in front of the CT detector and it is not disclosed a movable secondary collimation for cephalographic scanning placed in an X-ray sensor cassette posterior to the detector with respect to the source.
EP 1752099 teaches about a combined apparatus for panoramic and CT imaging modes, having adjustable magnification ratio and about a mounting part for attaching and detaching the suitable X-ray detector; however, it does not disclose cephalographic imaging and secondary collimation for cephalography.
EP 2198783 teaches about a simplified x-ray apparatus having a fixed secondary collimator placed at the extremity of a rotating arm, utilized for X-ray scanning in cephalography by a roto-translation movement of the same rotating arm and does not teach about a method for X-ray scanning in cephalography where the rotating arm is steady and the secondary collimator is moved thanks to the translational movement of the detector cassette where said secondary collimator is located in a position posterior to the detector with respect to the source.
Concerning the cone beam CT image acquisition and reconstruction various publication teach the method and algorithms, among which:
Med Phys. 2003 October; 30(10):2758-61.
Liu V, Lariviere N R, Wang G.—CT/Micro-CT Lab, Department of Radiology, University of Iowa, Iowa City, Iowa 52242, USA.
X-ray micro-CT with a displaced detector array: application to helical cone-beam reconstruction. In x-ray micro-CT applications, it is useful to increase the field of view by offsetting a two-dimensional (2D) detector array. In this technical note, we briefly review the methods for image reconstruction with an asymmetric 2D detector array, elaborate on the use of an associated weighting scheme in the case of helical/spiral cone-beam scanning, and perform a series of numerical tests to demonstrate helical cone-beam image reconstruction with such an arrangement.
Med Phys. 2002 July; 29(7):1634-6.—Wang G.—Department of Radiology, University of Iowa, Iowa City 52242, USA. ge-wang@uiowa.edu
X-ray micro-CT with a displaced detector array.
Because the sizes of samples differ in x-ray micro-CT applications, it is desirable to have a mechanism to change the field of view of a micro-CT scanner. A well-known way to double the diameter of the field of view is to displace a detector array by 50%. In this paper, we propose to displace a detector array by an amount of greater than 0% but less than 50% for a continuously adjustable field of view, and formulate a weighting scheme for artifact-free reconstruction. Then, we perform numerical simulation with the Shepp-Logan phantom to demonstrate the feasibility in fan-beam and cone-beam geometry.
Yu, L. Pelizzari, C. Pan, X. Riem, H. Munro, P. Kaissl, W.—Dept. of Radiol., Chicago Univ., Ill., USA
This paper appears in: Nuclear Science Symposium Conference Record, 2004 IEEE Publication Date: 16-22 Oct. 2004 Volume: 5—On page(s): 3249-3252 Vol. 5
ISSN: 1082-3654
E-ISBN: 0-7803-8701-5
Print ISBN: 0-7803-8700-7
INSPEC Accession Number: 8588605
Digital Object Identifier: 10.1109/NSSMIC.2004.1466376
Current Version Published: 1 Aug. 2005
Abstract
In many implementations of cone-beam CT in radiotherapy for target positioning, it is not uncommon that the maximum allowable field of view (FOV) cannot cover the patient because of the limited size of the flat-panel detector. In this situation, the measurements will contain truncated projections, leading to significant artifacts in reconstructed images. Asymmetric cone-beam configurations can be used for increasing the FOV size by displacing the detector panel to one side. From the data acquired with such an asymmetric configuration, the well-known algorithm developed by Feldkamp, Davis, and Kress (FDK) can be modified to reconstruct images. With increasing detector asymmetry, however, the modified FDK algorithm may produce significant aliasing artifacts. In this work, we develop a novel algorithm for image reconstruction in asymmetric cone-beam CT, which can generate images with improved numerical properties and allow for large detector asymmetry. We have employed the asymmetric configuration and the developed algorithm in a cone-beam CT system in radiotherapy to increase the FOV size. Preliminary phantom studies have been conducted to validate the asymmetric configuration and the proposed reconstruction algorithm.
Conebeam X-ray computed tomography with an offset detector array
Gregor, J.; Gleason, S. S.; Paulus, M. J.;
Dept. of Comput. Sci., Tennessee Univ., Knoxville, Tenn., USA
This paper appears in: Image Processing, 2003. ICIP 2003. Proceedings. 2003
International Conference on
On page(s): II-803-6 vol. 3
ISSN: 1522-4880
Print ISBN: 0-7803-7750-8
INSPEC Accession Number: 7978666
Digital Object Identifier: 10.1109/ICIP.2003.1246802
Current Version Published: 24 Nov. 2003
Abstract
Conventional X-ray computed tomography (CT) imaging is based on the assumption that the entire cross-section of an object is illuminated with X-rays at each view angle. When imaging a large object, a large detector array is thus needed. As an alternative, we propose to offset a normal sized detector array such that slightly more than one half of the required projection data is acquired. During reconstruction, the missing data is accounted for by means of an interpolation and weighting scheme. This algorithmic approach for extending the field of view, which we here present in the context of the popular Feldkamp algorithm, is simple but effective. Supportive experimental results are provided based on simulated phantom data as well as real data obtained from a MicroCAT™ which is a circular orbit microCT system for small animal imaging.
Comput Med Imaging Graph. 1996 January-February; 20(1):49-57.
Cone-beam CT from width-truncated projections.
Cho P S, Rudd A D, Johnson R H.
Source
Department of Radiation Oncology, University of Washington School of Medicine, Seattle 98195-6043, USA.
Abstract
In this paper we report cone-beam CT techniques that permit reconstruction from width-truncated projections. These techniques are variants of Feldkamp's filtered backprojection algorithm and assume quasi-redundancy of ray integrals. Two methods are derived and compared. The first method involves the use of preconvolution weighting of the truncated data. The second technique performs post-convolution weighting preceded by non-zero estimation of the missing information. The algorithms were tested using the three-dimensional Shepp-Logan head phantom. The results indicate that given an appropriate amount of overscan, satisfactory reconstruction can be achieved. These techniques can be used to solve the problem of undersized detectors.
Phys Med Biol. 2005 Apr. 21; 50(8):1805-20. Epub 2005 Apr. 6.
Exact fan-beam image reconstruction algorithm for truncated projection data acquired from an asymmetric half-size detector.
Leng S, Zhuang T, Nett B E, Chen G H.
Source
Department of Medical Physics, University of Wisconsin-Madison, 53704, USA.
Abstract
In this paper, we present a new algorithm designed for a specific data truncation problem in fan-beam CT. We consider a scanning configuration in which the fan-beam projection data are acquired from an asymmetrically positioned half-sized detector. Namely, the asymmetric detector only covers one half of the scanning field of view. Thus, the acquired fan-beam projection data are truncated at every view angle. If an explicit data rebinning process is not invoked, this data acquisition configuration will reek havoc on many known fan-beam image reconstruction schemes including the standard filtered backprojection (FBP) algorithm and the super-short-scan FBP reconstruction algorithms. However, we demonstrate that a recently developed fan-beam image reconstruction algorithm which reconstructs an image via filtering a backprojection image of differentiated projection data (FBPD) survives the above fan-beam data truncation problem. Namely, we may exactly reconstruct the whole image object using the truncated to data acquired in a full scan mode (2 pi angular range). We may also exactly reconstruct a small region of interest (ROI) using the truncated projection data acquired in a short-scan mode (less than 2 pi angular range). The most important characteristic of the proposed reconstruction scheme is that an explicit data rebinning process is not introduced. Numerical simulations were conducted to validate the new reconstruction algorithm.
A first problem not resolved from the prior art is to provide a solution that is at the same time protecting the CT detector, which is usually highly expensive and should not be equipped with manual detachment possibility, but allowing manual detachment of the panoramic detector, which is typically an area sensor with elongated size, and can be conveniently displaced in a position for Cephalography so allowing execution of panoramic and cephalometric imaging modes with a single sensor.
A second problem not adequately resolved from the prior art is that in order to achieve a flexibility of operation and an economic mechanical and electrical solution, it is preferable to have an arrangement provided with a single movement capable of performing all the necessary detector positioning as well as the scanning movement required for Cephalography.
Among the necessary detector positioning is included the partial offset positioning required to perform a particular CT imaging mode defined in literature as “extended view”, where thanks to the acquisition of multiple images by an offset rotation around the object an extended portion of the region of interest of the patient is reconstructed.